As air passes over the evaporator coil inside an air handler, such as one used with a split air conditioning or heat pump system, condensate forms on the coil. This condensate descends from the coil into the primary drain pan inside the air handler. In case the primary pan overflows, contractors often install a secondary, or emergency, drain pan under the air handler.
A secondary drain pan is typically made of plastic or metal and is rectangular or round in shape. Common pan sizes range from 18″×46″ to 36″×60″ to 34″×79″. Such a pan typically has side walls that define a basin that contains a waterway that allows drainage to exit through a drainage hole. The secondary pan rests on a platform or hangs under suspended equipment.
Secondary drain pans should be maneuverable enough to fit through attic or crawlspace doors. For cost, handling, and code considerations, plastic pans are generally made from a minimum 0.065″ thick material, and sturdier pans may be 0.125″ thick, or more. Sheet metal pans generally have a minimum thickness of 0.0236″ (24 gage).
A variety of methods and materials, some referred to as risers, are employed in the field in order to elevate the installed equipment above the level of any water that may collect in the pan. Equipment may be elevated further to provide proper drainage pitch for the primary drain line and easy access to equipment panels. This elevation is achieved by using substantial risers, by constructing a platform under the drain pan, or by hanging the unit from the ceiling trusses. As discussed in my patent application Ser. No. 11/320,992, filed Dec. 29, 2005, one improvement to the art is a drain pan with integrated risers that provide structural support for the air handler. When formed of plastic, that pan requires thicker material than ordinary pans in order to meet structural requirements and to withstand potentially high attic temperatures (140° F.). Yet the pan still must be set on a plywood platform for support. Also, pans in general are too flexible to hang under suspended air handlers without support as well.